diff --git a/lib/build/amodro-trace/read/es.js b/lib/build/amodro-trace/read/es.js
index f13e49a5..b59d8f5e 100644
--- a/lib/build/amodro-trace/read/es.js
+++ b/lib/build/amodro-trace/read/es.js
@@ -5,7 +5,7 @@ const transformSync = require('@babel/core').transformSync;
  * @param {string} fileName 
  * @param {string} fileContents 
  * @param {{} | boolean} inputSourceMap 
- * @returns {{ code: string, map: string, ast: any }}
+ * @returns {babel.BabelFileResult}
  */
 module.exports = function es(fileName, fileContents, inputSourceMap) {
   return transformSync(fileContents, {
diff --git a/lib/build/bundled-source.js b/lib/build/bundled-source.js
index 32cbdaaf..16da4baf 100644
--- a/lib/build/bundled-source.js
+++ b/lib/build/bundled-source.js
@@ -14,11 +14,12 @@ exports.BundledSource = class {
     this.includedIn = null;
     this.includedBy = null;
     this._contents = null;
+    this._transformedSourceMap = undefined;
     this.requiresTransform = true;
   }
 
   get sourceMap() {
-    return this.file.sourceMap;
+    return this._transformedSourceMap || this.file.sourceMap;
   }
 
   get path() {
@@ -215,6 +216,7 @@ exports.BundledSource = class {
 
       if (cache) {
         this.contents = cache.contents;
+        this._transformedSourceMap = cache.transformedSourceMap;
         deps = cache.deps;
       } else {
         let contents;
@@ -239,7 +241,7 @@ exports.BundledSource = class {
 
             contents = result.code;
             if (result.map) {
-              this.file.sourceMap = result.map; // save updated source map
+              this._transformedSourceMap = result.map;
             }
           } catch (e) {
             logger.error('Could not convert to AMD module, skipping ' + modulePath);
@@ -279,7 +281,8 @@ exports.BundledSource = class {
         if (useCache && hash) {
           Utils.setCache(hash, {
             deps: deps,
-            contents: this.contents
+            contents: this.contents,
+            transformedSourceMap: !this._transformedSourceMap ? undefined : this._transformedSourceMap // avoid serializing `null`
           });
         }
       }
diff --git a/spec/lib/build/bundled-source.spec.js b/spec/lib/build/bundled-source.spec.js
index efc98835..ca8be720 100644
--- a/spec/lib/build/bundled-source.spec.js
+++ b/spec/lib/build/bundled-source.spec.js
@@ -881,5 +881,100 @@ module.exports = require('./bar.js');
       expect(Utils.getCache).toHaveBeenCalled();
       expect(Utils.setCache).not.toHaveBeenCalled();
     });
+
+    it('transform saves transformed source map to cache', () => {
+      let file = {
+        path: path.resolve(cwd, 'node_modules/foo/bar/lo.js'),
+        contents: "export {default as t} from './t.js';",
+        sourceMap: {"version":3,"file":"lo.js","sourceRoot":"","sources":["lo.ts"],"names":[],"mappings":"AAAA,OAAO,EAAC,OAAO,IAAI,CAAC,EAAC,MAAM,QAAQ,CAAA"}
+      };
+
+      Utils.getCache = jasmine.createSpy('getCache').and.returnValue(undefined);
+      Utils.setCache = jasmine.createSpy('setCache');
+
+      let bs = new BundledSource(bundler, file);
+      bs._getProjectRoot = () => 'src';
+      bs.includedBy = {
+        includedBy: {
+          description: {
+            name: 'foo',
+            mainId: 'foo/index',
+            loaderConfig: {
+              name: 'foo',
+              path: '../node_modules/foo',
+              main: 'index'
+            },
+            browserReplacement: () => undefined
+          }
+        }
+      };
+      bs._getLoaderPlugins = () => [];
+      bs._getLoaderConfig = () => ({paths: {}});
+      bs._getUseCache = () => true;
+
+      let deps = bs.transform();
+      let contents = "define('foo/bar/lo',[\"exports\", './t'], function (_exports, _t) {  \"use strict\";  _exports.__esModule = true;  _exports.t = void 0;  _t = _interopRequireDefault(_t);  _exports.t = _t.default;  function _interopRequireDefault(e) { return e && e.__esModule ? e : { default: e }; }});";
+      let transformedSourceMap = {version:3,file:undefined,names:['_t', '_interopRequireDefault', '_exports', 't', 'default', 'e', '__esModule'],sourceRoot:undefined,sources:['lo.ts'],sourcesContent:[undefined],mappings:";;;;;EAAAA,EAAA,GAAAC,sBAAA,CAAAD,EAAA;EAA2BE,QAAA,CAAAC,CAAA,GAAAH,EAAA,CAAAI,OAAA;EAAA,SAAAH,uBAAAI,CAAA,WAAAA,CAAA,IAAAA,CAAA,CAAAC,UAAA,GAAAD,CAAA,KAAAD,OAAA,EAAAC,CAAA;AAAA",ignoreList:[]};
+
+      expect(deps).toEqual(['foo/bar/t']);
+      expect(bs.requiresTransform).toBe(false);
+      expect(bs.contents.replace(/\r|\n/g, ''))
+        .toBe(contents);
+      expect(bs.sourceMap).toEqual(transformedSourceMap)
+
+      expect(Utils.getCache).toHaveBeenCalled();
+      expect(Utils.setCache).toHaveBeenCalled();
+      expect(Utils.setCache.calls.argsFor(0)[1].deps).toEqual(['./t']);
+      expect(Utils.setCache.calls.argsFor(0)[1].contents.replace(/\r|\n/g, '')).toBe(contents);
+      expect(Utils.setCache.calls.argsFor(0)[1].transformedSourceMap).toEqual(transformedSourceMap);
+    });
+
+    it('transform uses cache for transformed source map', () => {
+      let file = {
+        path: path.resolve(cwd, 'node_modules/foo/bar/lo.js'),
+        contents: "export {default as t} from './t.js';",
+        sourceMap: {"version":3,"file":"lo.js","sourceRoot":"","sources":["lo.ts"],"names":[],"mappings":"AAAA,OAAO,EAAC,OAAO,IAAI,CAAC,EAAC,MAAM,QAAQ,CAAA"}
+      };
+
+      let contents = "define('foo/bar/lo',[\"exports\", './t'], function (_exports, _t) {  \"use strict\";  _exports.__esModule = true;  _exports.t = void 0;  _t = _interopRequireDefault(_t);  _exports.t = _t.default;  function _interopRequireDefault(e) { return e && e.__esModule ? e : { default: e }; }});";
+      let transformedSourceMap = {version:3,file:undefined,names:['_t', '_interopRequireDefault', '_exports', 't', 'default', 'e', '__esModule'],sourceRoot:undefined,sources:['lo.ts'],sourcesContent:[undefined],mappings:";;;;;EAAAA,EAAA,GAAAC,sBAAA,CAAAD,EAAA;EAA2BE,QAAA,CAAAC,CAAA,GAAAH,EAAA,CAAAI,OAAA;EAAA,SAAAH,uBAAAI,CAAA,WAAAA,CAAA,IAAAA,CAAA,CAAAC,UAAA,GAAAD,CAAA,KAAAD,OAAA,EAAAC,CAAA;AAAA",ignoreList:[]};
+
+      Utils.getCache = jasmine.createSpy('getCache').and.returnValue({
+        deps: ['./t'],
+        contents,
+        transformedSourceMap
+      });
+      Utils.setCache = jasmine.createSpy('setCache');
+
+      let bs = new BundledSource(bundler, file);
+      bs._getProjectRoot = () => 'src';
+      bs.includedBy = {
+        includedBy: {
+          description: {
+            name: 'foo',
+            mainId: 'foo/index',
+            loaderConfig: {
+              name: 'foo',
+              path: '../node_modules/foo',
+              main: 'index'
+            },
+            browserReplacement: () => undefined
+          }
+        }
+      };
+      bs._getLoaderPlugins = () => [];
+      bs._getLoaderConfig = () => ({paths: {}});
+      bs._getUseCache = () => true;
+
+      let deps = bs.transform();
+      expect(deps).toEqual(['foo/bar/t']);
+      expect(bs.requiresTransform).toBe(false);
+      expect(bs.contents.replace(/\r|\n/g, ''))
+        .toBe(contents);
+      expect(bs.sourceMap).toEqual(transformedSourceMap);
+
+      expect(Utils.getCache).toHaveBeenCalled();
+      expect(Utils.setCache).not.toHaveBeenCalled();
+    });
   });
 });